1. Field of the Invention
The present invention relates to an electrophotographic recording apparatus and particularly to a sheet transferring mechanism of an image transcription unit provided in the electrophotographic recording apparatus.
Recently, an electrophotographic recording apparatus such as an electrically printer or duplicator is widely used for electrically recording letters and/or images onto recording sheets. The electrophotographic recording apparatus, which will be simply called as the "recording apparatus" hereinafter, comprises: a photoconductive drum; a charger for electro-statically charging a surface of the photoconductive drum; a light irradiating unit for irradiating an optical image onto the surface of the photoconductive drum so that a latent image is formed on the surface of the charged photoconductive drum; a developing unit for developing the latent image by using toner so that a toner image appears on the surface of the photoconductive drum; an image transcription unit for transcribing the toner image onto a recording sheet; a sheet transferring mechanism for transferring the recording sheets into and out of the image transcription unit one by one, synchronizing with the rotation of the photoconductive drum; an image fixing unit for fixing a toner image transcribed on each recording sheet; and a cleaner for cleaning the toner left on the surface of the photoconductive drum after the transcription of the toner image on the recording sheet is over. Furthermore, the sheet transferring mechanism provided around the image transcription unit includes a sheet intruding guide and a sheet extruding guide. The sheet intruding guide is located at a sheet entrance of the image transcription unit for intruding the recording sheet into the image transcription unit and a sheet extruding guide is located at a sheet exit of the image transcription unit for extruding the recording sheet after the toner image is transcribed on the recording sheet.
In the recording apparatus, a gap provided between the sheet intruding guide and the photoconductive drum and a gap provided between the sheet extruding guide and the photoconductive drum are very significant. The values of these gaps are determined in consideration of various conditions such as the rotating speed of the photoconductive drum, the size and the physical property of the recording sheet, an intruding point of the recording sheet to the rotating surface of the photoconductive drum, and an intruding angle of the recording sheet to the surface of the rotating photoconductive drum at the intruding point. If the gaps vary with the rotation of the photoconductive drum as much as to exceed allowable values, the tip and/or the end of the intruded recording sheet is fluttered in the image transcription unit, causing the toner image transcribed on the recording sheet to lack in uniformity, which results in exerting a harmful influence on the quality of the recorded image on the recording sheet.
Furthermore, in the recording apparatus, the units such as the photoconductive drum and the cleaner are unified into one unit called a "drum unit". This is for allowing the users of the recording apparatus to easily exchange these units when some of the units are worn. Actually, the drum unit is exchanged periodically particularly in consideration of the life of the photoconductive drum. However, the image transcription unit is hard to be unified into the drum unit because dust such as the toner powder worn off from the surface of the photoconductive drum and the sheet trash from the recording sheet are left in the image transcription unit before the life of the image transcription unit is over. Therefore, the image transcription unit must be periodically cleaned up for maintaining its correct operation. Accordingly, whenever the process unit is exchanged or the image transcription unit is cleaned, the great attention must be payed for keeping the gaps in the designated value, which is not always easy to be done. Therefore, it can be said that if the gaps can be always kept constant easily even though the exchange or the cleaning is carried out by the users, the recording apparatus comes to be used more widely.
2. Description of the Prior Art
A side view of an image transcription unit and the prior art sheet transferring mechanism around the image transcription unit are illustrated in FIG. 1. In FIG. 1, a photoconductive drum 1, which will be simply called a "drum 1" hereinafter, is rotated counterclockwise around an axis 1' as shown in a rotational direction D.sub.1. An image transcription unit 3 is placed just under the drum 1, having a space 4, and the toner image on the drum 1 is transcribed onto the recording sheet when the recording sheet passes through the space 4. The recording sheet is sent to the space 4 through a sheet intruding guide 2 composed of a guide frame 5 and a plastic film 6 fixed on the guide frame 5. The upper surface of the plastic film 6 is parallel to the axis 1' and the tip edge, toward the surface of the drum 1, of the plastic film 6 is also parallel with the axis 1' and provides a gap A to the surface of the drum 1. The recording sheet sent to the upper surface of the plastic film 6 is guided to the rotating surface of the drum 1 through the gap A and rolled into the space 4 in a state that the recording sheet sticks fast to the rotating surface of the drum 1. Therefore, when the recording sheet is arrived to the space 4, the toner image on the surface of the rotating drum 1 is transcribed to the surface of the recording sheet by the electrostatic force provided from the image transcription unit 3.
The image transcription unit 3 is composed of: a corona charger 8 which produces an electrostatic force by which the toner image on the surface of the rotating drum 1 is transcribed to the surface of the recording sheet; and a corona discharger 9 for discharging the charge on the recording sheet so that the recording sheet having been stuck onto the surface of the drum 1 can be easily peeled off from the surface of the drum 1 after the transcription is over. The corona charger 8 and the corona discharger 9 have wires 7 and 7' respectively for making them operate properly, and they are separated by a separating wall 77. On the upper edge of the separating wall 77, a sheet extruding guide 10 consisting of a plurality of guide pieces is fixed, which is illustrated by a partial plan view of the image transcription unit 3 in FIG. 2. The guide pieces are arranged in a plane parallel to the axis 1' aslant to the sheet transferring direction D so that halves of them are arranged in different direction having the same angle from the direction D.sub.2 as shown in FIG. 2. This is for extruding the recording sheet from the sheet extruding guide 10 straight along the direction D.sub.2.
In FIG. 1, the axis 1', the sheet intruding guide 2, the image transcription unit 3 and the sheet extruding guide 10 are all fixed to a base frame, which is not depicted in FIG. 1, of the recording apparatus. Therefore, it is very hard to maintain the values of the gaps A and B in high accuracy under a condition that the drum 1 must be periodically removed for exchanging or for cleaning the inside of the image transcription unit 3. For example, even though the gaps A and B are determined so as to be 0.5 mm.+-.0.2 mm and 1.5 mm.+-.0.3 mm respectively for obtaining the high quality of the transcribed image on the recording sheet, it will be very hard to set the gaps A and B with the above tolerance. Because, usually the eccentricity of the drum 1 is 0.15 mm, the positioning accuracy of the wire 7 or 7' is 0.1 mm, the positioning accuracy of the sheet intruding guide 2 is 0.1 mm, and furthermore there is a little variation of the radius of the drum 1. It cannot be said impossible to manufacture the recording apparatus with high accuracy; however, the manufacturing cost becomes extremely high, and this high cost cannot be allowed in the commercial price of the recording apparatus.